The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In data center applications it is often desirable to share one battery in connection with two or more uninterruptible power supply (UPS) systems. An example of one such system in shown in FIG. 1, where two UPSs are sharing one battery. Typically the two UPSs are coupled in parallel across the three phase power distribution (with or without a neutral line) at an input and across a common load connection at the output. Each UPS is connected to the positive and negative terminals of a battery. The input is the power distribution connection on the Rectifier modules (not drawn in FIG. 1) while the output comes from the output junction among the UPSs (drawn in FIG. 1). In one specific implementation, one potential drawback may exist when the parallel coupled UPSs are sharing a common battery. The drawback is that a small DC reference voltage can be generated between the DC− busses of the two UPSs under some circumstances. This “imbalance” can in some circumstances result in a relatively large current flowing on the DC− busses between the two UPSs. This current is indicated by arrow “A” in FIG. 1. The current flowing between the two DC− busses may be quite large, for example in some instances up to several hundred amps in magnitude. Such a situation is undesirable because it can result in poor current sharing among the rectifier circuits of the two UPSs, for example a difference as much as 30% and 70% between the two rectifiers of the two UPSs. This can significantly limit the current which is able to be delivered by one of the UPSs to its associated load.
FIG. 2 illustrates how this imbalance may come about. This is due to tolerances of the bleed resistors Rhigh and Rlow which are present in each of the rectifier, inverter and booster modules in every UPS. These resistors typically are 5% tolerance resistors, and are all connected in parallel. With a nominal DC reference of 780V, a neutral imbalance of 1V with respect to the nominal value will produce 389V on the DC+ line and −391V on the DC− line, with respect to DC+=390V nominal and DC−=−390V nominal. This imbalance is directly reported to the DC− bar potential due to a neutral balancing algorithm that is employed in UPSs of the assignee of the present application. The neutral balancing algorithm has the task of aligning all internal neutral potentials to the neutral line of a three phase star input (VIN3STAR), which is also typically tied to ground. The aligned internal neutrals of the two UPS systems are shown in FIG. 3, where under ideal conditions Rhigh=Rlow and Neutral UPS1 equals UPS2 inside each UPS system. Under this situation no current will flow in the DC− busses between the two UPS systems.
The neutral balancing algorithm described above may involve initially summing the three phase inputs and then dividing the sum by three for internal digital signal processing purposes. Considering that the input phases are measured with respect to neutral, and due to the fact that the phases have a 3° harmonic overlap, the summed and divided signal needs to be filtered to eliminate any possible residual of the fundamental and all harmonics. From theory, the sum of the three input phases eliminates the fundamental (tri-phase) but not the third harmonic. The filter used can remove the fundamental as well as all harmonics. In particular, this can be accomplished using a moving average filter with the length of a fundamental period. The neutral balancing algorithm attempts to move this value to 0→VNX (DC offset)VIN3STAR=0. In this example “VNX (DC offset)VIN3STAR” refers to the potential of the internal neutral of UPS X (only the DC component) measured with respect to the center star of the tri-phase input. In those cases in which VIN3STAR allows the DC offset of the neutral with respect to GND=0V to be obtained (VIN3STARGND=0V), because the center star is connected to GND, this means that VNX (DC offset)GND=0V. In this example VIN3STAR refers to the potential of the center star tri-phase input. VIN3STARGND refers to the potential of the center star tri-phase input measured with respect to GND. If this is 0V, this means that the center star tri-phase input is tied to GND. VNX (DC offset)GND refers to the potential of the internal neutral of UPS X (only the DC component) measured with respect to GND. A gain may then be applied to the filtered result. This parameter manages the algorithm loop gain. The result is then summed to an offset value obtained using a previously implemented calibration procedure. The result is used to feed the error input of a proportional integral (PI) regulator and its output is added to a PWM (pulse width modulated) modulator signal.
The challenge described above comes about when two UPS systems are configured to share a common battery. Such a configuration is somewhat common in data center environments because it can be a significant cost savings to configure two UPS systems so that they share a common battery. Under this situation, however, when a difference in resistance exists between Rhigh and Rlow, shown in FIG. 4, and the resulting neutral imbalance between the internal neutrals of the two UPS systems is corrected by the neutral correction algorithm mentioned above, such neutral balancing will also result in shifting the DC− level (and of course the DC+ level as well) of one of the UPS systems. This “shifting” is illustrated in FIG. 5 with arrow “B” denoting the magnitude of the shift between the DC− potential of a first UPS system and the DC− potential of a second UPS system. However, this imbalance between the two DC− potentials for the two UPS systems is what results in the relatively large current flow on the DC− bus from one UPS system to the DC− bus of the other UPS system. The large current flow can result in one UPS system receiving significantly more current than the other UPS system.